1. Field of the Invention
The invention relates generally to improved drive systems for vehicles and more particularly to an improved hydrostatic drive system for a combine particularly adapted for use in hilly terrain.
2. Prior Art
It is well known in the art of hydrostatic drive systems to include motors of variable displacement whose torque output can be controlled. Examples of such drives are shown in U.S. Pat. Nos. 3,734,225; 3,637,036; 3,595,334 and 3,587,765. Of these patents '225 and '334 use pressure responsive means for varying motor displacement and the torque output of the motor.
Auxiliary drive systems for combines are shown in U.S. Pat. Nos. 4,140,196 and 3,736,732, but using fixed displacement motors. Auxiliary drive systems having electrical controls are shown in U.S. Pat. Nos. 4,177,870; 4,027,738 and 3,894,606, but using fixed displacement motors.
Earlier attempts by the assignee herein to develop automatic shifting from two wheel drive to four wheel drive for combines brought several problems to light. Excessive pressure spikes were induced in the hydrostatic system when shifting from two to four wheel drive and back to two wheel drive. Such pressure spikes can damage system components such as hoses and seals. Perhaps the overriding problem with earlier systems was that the machine operator experienced excessive deceleration and acceleration in shifting from two to four wheel drive and four to two wheel drive, respectively.
In developing an effective drive system for combines, special factors are involved such as the increasing load on the combine as grain is collected in the grain tank during harvesting. Where the combine is destined for principal use in harvesting crops on hilly terrain (such as wheat in eastern Washington State), the function of a drive system takes on particularly important considerations of operator and vehicle safety. That is, the degree of slope traversed by so-called "hillside" combines can require maximum tractive effort not only to move the combine, but to assist in preventing downhill side slip of the machine. When such factors are involved, safety of operation is enhanced where the drive system can automatically respond to conditions or to the requirements of the operator.